PROJECT SUMMARY Glioblastoma is the most common form of malignant brain tumor, with a dismal 5-year survival rate of 5.0% after multimodal therapy. The highly aggressive and infiltrative properties of glioblastoma cells complicate treatment by evading surgical resection and local chemotherapies. Consequently, investigation into mechanisms of glioblastoma proliferation, migration and invasion are needed to develop improved therapeutics and increase the efficacy of current treatments. The low-density lipoprotein receptor-related protein 1B (LRP1B) is a large endocytic receptor that has been identified as one of the most commonly deleted genes across human cancers. Recently, deletion of LRP1B was shown to be significantly associated with poor overall survival and poor progression-free survival in glioblastoma patients. Despite its prevalence across human cancers, the mechanism(s) by which LRP1B acts as a tumor suppressor are largely unknown. A published study from our lab demonstrated that LRP1B undergoes regulated intramembrane proteolysis, a process that involves the sequential proteolysis of a protein to release a soluble extracellular domain and intracellular domain. The LRP1B intracellular domain (LICD) was found to localize to the nucleus and inhibit anchorage- independent growth. Additionally, LRP1B has been found to interact with proteins involved in Wnt signaling and a number of transcription factors. Preliminary studies have found strong negative correlations of LRP1B mRNA expression with the expression of Wnt signaling activators and target genes in glioblastoma patients. With this support, the central hypothesis of this project is that the deletion of LRP1B promotes glioblastoma progression through increased Wnt signaling. Our hypothesis will be tested in the following specific aims. 1) Determine if the LRP1B intracellular domain is sufficient to attenuate glioblastoma proliferation and migration. 2) Define the mechanism(s) by which LRP1B attenuates cellular proliferation and migration. Specific Aim 1 will investigate the LRP1B intracellular domain and define the specific amino acid sequences that contribute to its tumor suppressor function. To complement these studies, Specific Aim 2 will investigate the protein and DNA interactions of the LRP1B intracellular domain, and will quantify global proteomic and phosphorylation changes that occur upon LRP1B expression. Together these studies will give insight into mechanisms by which LRP1B mediates tumor suppression. These aims will be accomplished using a variety of strategies, including mass spectrometry-based proteomics, chromatin immunoprecipitation, next-generation sequencing and molecular biology techniques. Mechanistic knowledge of LRP1B has the potential to enable better therapeutic choices and reveal specific targets for rational drug design. Additionally, these insights reach beyond glioblastoma treatment, given the prevalence of LRP1B deletions across human cancers and the role of LRP1B in atherosclerosis and Alzheimer?s disease.